1. Field of the Invention
This invention relates to foil for regenerators of regenerative gas cycle machinery.
2. Description of Prior Art
Regenerative gas cycle machines are a class of machinery that includes Stirling cycle engines and Stirling cycle, Gifford-McMahon, Vuilleumier, Solvay and pulse tube refrigerators. A regenerator is a critical component of all regenerative gas-cycle machines. The regenerator acts as a thermal sponge. Fluid passing back and forth through the regenerator leaves heat in the regenerator matrix in one direction of flow and picks up that heat as it passes back through the regenerator in the opposite direction.
Stacks of wire-mesh screens, wire felt materials, and beds of packed metal powder have been widely used as regenerators in gas cycle machinery because the materials are primarily used for other purposes, are produced in quantity, and are readily available in the marketplace. However, none of those materials is specifically designed to fulfill the special function of a regenerator. Regenerators fabricated from those materials all contain random fluid flow passages in the spaces between wires or grains of powder. The flow passages are of varying width, and a significant portion of the void volume in those regenerator is in spaces in which there is little or no fluid flow and thus little opportunity for heat transfer between the fluid and the regenerator matrix material. One advantage of those prior art materials was that the regenerator permitted lateral flows as well as flows in the overall direction of flow in the regenerator. That permitted imbalances in flow at different points in each cross section of the regenerator to be equalized by natural cross-flows. However, these materials contain no means for dynamically redistributing fluid laterally relative to the overall direction of flow in the regenerator.
Spaced layers of foil have also been used as the matrix material in regenerators in gas cycle machinery. Sheets of foil can be etched to create grooves on the surface of the foil. Foil can also be shaped by crimping or dimpling it, which avoids the loss of material in the etching process, but those techniques have not been sufficiently precise to produce acceptable regenerators. Moreover, solid layers of foil prevent cross-flows necessary to rebalance overall flow distribution over a cross section of the regenerator as fluid moves through it.
Etched foil regenerators used heretofore have partially solved the problem of flow passage width; if the foil is prepared carefully, flow passages are close to the same width throughout the regenerator. Perforations in etched foil have also permitted cross-flows, as in screen, felt and packed powder regenerators. In practice, performance of prior art foil regenerators has generally been disappointing.
Laboratory work with prior art foil regenerators shows that they offer lower pressure drop than felted material, stacked screens or packed powder, the standard regenerator materials. Computer models suggest that prior art foil regenerators should also provide good heat transfer, and, overall, superior performance.
Disappointing performance of prior art foil regenerators is due in part to inadequate heat transfer between the fluid and the foil. When fluid passes straight through the regenerator from one end to the other, the time that the fluid spends in transit is minimized, limiting the time during which heat transfer can take place. Moreover, boundary layers develop as fluid flows through the regenerator, impeding heat transfer.
Stainless steel can be used in foil regenerators operating down to about 30 Kelvins, but for regenerators to be used in coolers that reach temperatures below about 30 Kelvins, other, more expensive materials with better low-temperature heat capacity are required. Those materials include alloys of rare earth materials. Some of those materials can be formed into foil, but it is not economical to etch that foil to produce perforated regenerator foil because too much of the expensive material would be etched away and thus wasted.
Even with relatively inexpensive materials such as lead and its alloys, etching grooves on the material is not practical because the material is already relatively weak and etching grooves in the material weakens it further, exacerbating problems of handling and assembling it into a regenerator without damaging it.